The applications of Microelectro-mechanical System (MEMS) have gained popularity in many consumer electronic products. For example, Wii from Nintendo uses a MEMS-based three-axial acceleration sensor to work with a wireless controller to achieve the highly creative entertainment.
MEMS is an intelligent micro-system, usually including sensor, processor or enabler so that a single chip or a multi-chip set can integrate a plurality of electronic, mechanical, optical, chemical, biological and magnetic functions. MEMS is widely applied to various industries, such as, manufacturing, automation, information and communication, aerospace, transportation, construction, environmental protection, agriculture, forestry, fishery and farming.
MEMS requires appropriate analog-to-digital converters (ADC) to convert the MEMS analog output signal to digital output signal for subsequent information processing by the digital processor, where Σ-Δ (sigma-delta) ADC is a common choice of ADC.
FIG. 1 shows a schematic view of a functional diagram of the conventional apparatus for converting MEMS inductive capacitance. As shown in FIG. 1, an apparatus 1 for converting MEMS inductive capacitance to digital signal includes a MEMS sensor 10, a sensor amplifier 20, a bias circuit 30 and ADC 40, where sensor amplifier 20 amplifies the output signal from MEMS sensor 10, and ADC 40 converts the amplified signal into digital signals. Bias circuitry 30 provides a suitable bias voltage for sensor amplifier 20 and ADC 40.
FIG. 2 shows a detailed view of FIG. 1. As shown in FIG. 2, the electric model of MEMS sensor 10 shows a MEMS capacitor CS and bias input impedance R. MEMS capacitor CS has a capacitance change ΔCS about 50 f caused by the external change. Under the condition of bias voltage Vbias=10V, MEMS capacitor CS voltage change ΔVCS is about 1 mV, which is amplified by sensor amplifier 20 and input to ADC 40. Take a one-stage Σ-Δ ADC as an example. ADC 40 includes a first-stage converter circuit 41 and a comparator 45, where first-stage converter circuit 41 further includes a subtracter 42, an adder 43, a delay relay 44 and a digital-to-analog converter (DAC) 46. DAC 46 converts the digital output signal Vout from comparator 45 into analog signal. Subtracter 42 finds the difference between the output signal of sensor amplifier 20 and the output signal of DAC 46. Adder 43 adds the output signal of delay relay 44 to the difference, and outputs to delay relay 44 so as to complete the entire ADC operation. As Σ-Δ ADC is a commonly known technique, the above description is only to highlight the key points.
In addition, in a conventional Σ-Δ ADC structure, to improve the resolution of ADC, a structure with a plurality of serial stages is usually used. That is, the output signal of first-stage converter circuit 41 can be passed to the next stage converter circuit, and the last stage converter circuit is connected to the comparator.
However, the conventional technique has the drawback of requiring a bias circuit able to generate a high bias voltage (about 10V or higher) so as to increase the sensing sensitivity to MEMS. Because the sensitivity of MEMS increases as the bias voltage increases, it is a difficult challenge for the general IC fabrication process, and also difficult to integrate into the other existing function blocks operating at low voltage.
Another drawback of the conventional technique is requiring a high quality amplifier to amplify the 1-mV MEMS output signal to the voltage range processable by ADC. As the amplifier requires a large size chip area, the chip cost increases and the offset, gain and noise of the amplifier will also increase the signal error.
Hence, it is imperative to devise an apparatus able to use ADC to directly convert the low level output signal of MEMS to digital signal to save the sensor amplifier and the bias circuit to facilitate a smaller-size chip area.